The present invention relates to electrochemical fuel cell stacks. In particular, the invention provides an electrochemical solid polymer fuel cell stack with improved reactant manifolding and sealing.
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells typically employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The membrane, in addition to being ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant streams from each other.
The MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the MEA. Surfaces of the separator plates that contact an electrode are referred to as active surfaces. The separator plates may have grooves or open-faced channels formed in one or both surfaces thereof, to direct the fuel and oxidant to the respective contacting electrode layers, namely, the anode on the fuel side and the cathode on the oxidant side. Such separator plates are known as flow field plates, with the channels, which may be continuous or discontinuous between the reactant inlet and outlet, being referred to as flow field channels. The flow field channels assist in the distribution of the reactant across the electrochemically active area of the contacted porous electrode. In some solid polymer fuel cells, flow field channels are not provided in the active surfaces of the separator plates, but the reactants are directed through passages in the porous electrode layer. Such passages may, for example, include channels or grooves formed in the porous electrode layer or may just be the interconnected pores or interstices of the porous material.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, an active surface of the separator plate faces and contacts an electrode and a non-active surface of the plate may face a non-active surface of an adjoining plate. In some cases, the adjoining non-active separator plates may be bonded together to form a laminated plate. Alternatively both surfaces of a separator plate may be active. For example, in series arrangements, one side of a plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell, with the separator plate functioning as a bipolar plate. Such a bipolar plate may have flow field channels formed on both active surfaces.
The fuel stream that is supplied to the anode separator plate typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
A fuel cell stack typically includes inlet ports and supply manifolds for directing the fuel and the oxidant to the plurality of anodes and cathodes respectively. The stack often also includes an inlet port and manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack. The stack manifolds, for example, may be internal manifolds, which extend through aligned openings formed in the separator layers and MEAs, or may comprise external or edge manifolds, attached to the edges of the separator layers.
Conventional fuel cell stacks are sealed to prevent leaks and inter-mixing of the fuel and oxidant streams. Fuel cell stacks typically employ fluid tight resilient seals, such as elastomeric gaskets between the separator plates and membranes. Such seals typically circumscribe the manifolds and the electrochemically active area. Applying a compressive force to the resilient gasket seals effects sealing.
Fuel cell stacks are compressed to enhance sealing and electrical contact between the surfaces of the plates and the MEAs, and between adjoining plates. In conventional fuel cell stacks, the fuel cell plates and MEAs are typically compressed and maintained in their assembled state between a pair of end plates by one or more metal tie rods or tension members. The tie rods typically extend through holes formed in the stack end plates, and have associated nuts or other fastening means to secure them in the stack assembly. The tie rods may be external, that is, not extending through the fuel cell separator plates and MEAs, however, external tie rods can add significantly to the stack weight and volume. It is generally preferable to use one or more internal tie rods which extend between the stack end plates through openings in the fuel cell separator plates and MEAs as, for example, described in U.S. Pat. No. 5,484,666. Typically springs, hydraulic or pneumatic pistons, pressure pads or other resilient compressive means are utilized to cooperate with the tie rods and end plates to urge the two end plates towards each other to compress the fuel cell stack components.
The passageways which fluidly connect each electrode to the appropriate stack supply and/or exhaust manifolds typically comprise one or more open-faced fluid channels formed in the active surface of the separator plate, extending from a reactant manifold to the area of the plate which corresponds to the electrochemically active area of the contacted electrode. In this way, for a flow field plate, fabrication is simplified by forming the fluid supply and exhaust channels on the same face of the plate as the flow field channels. However, such channels may present a problem for the resilient seal, which is intended to fluidly isolate the other electrode (on the opposite side of the ion exchange membrane) from this manifold. Where a seal on the other side of the membrane crosses over open-faced channels extending from the manifold, a supporting surface is required to bolster the seal and to prevent the seal from leaking and/or sagging into the open-faced channel. One solution adopted in conventional separator plates is to insert a bridge member that spans the open-faced channels underneath the resilient seal. The bridge member preferably provides a sealing surface that is flush with the sealing surface of the separator plate so that a gasket-type seal on the other side of the membrane is substantially uniformly compressed to provide a fluid tight seal. The bridge member also prevents the gasket-type seal from sagging into the open-faced channel and restricting the fluid flow between the manifold and the electrode. Instead of bridge members, it is also known to use metal tubes or other equivalent devices for providing a continuous sealing surface around the electrochemically active area of the electrodes (see, for example, U.S. Pat. No. 5,570,281), whereby passageways, which fluidly interconnect each electrode to the appropriate stack supply or exhaust manifolds, extend laterally within the thickness of a separator or flow field plate, substantially parallel to its major surfaces.
Conventional bridge members are affixed to the separator plates after the plates have been milled or molded to form the open-faced fluid channels. One problem with this solution is that separate bridge members add to the number of separate fuel cell components that are needed in a fuel cell stack. Further, the bridge members are typically bonded to the separator plates, so care must be exercised to ensure that the relatively small bridge members are accurately installed and that the bonding agent does not obscure the manifold port. It is also preferable to ensure that the bridge members are installed substantially flush with the sealing surface of the separator plate. Accordingly, the installation of conventional bridge members on separator plates adds significantly to the fabrication time and cost for manufacturing separator plates for fuel cell assemblies. Therefore, it is desirable to obviate the need for such bridge members, and to design an electrochemical fuel cell stack so that the fluid reactant streams are not directed between the separator plates and MEA seals.
In the present approach, passageways fluidly interconnecting an anode to a fuel manifold, or interconnecting a cathode to an oxidant manifold, in an electrochemical fuel cell stack are formed between the non-active surfaces of a pair of adjoining separator plates. The passageway then extends through one or more ports penetrating the thickness of one of the plates thereby fluidly connecting the manifold to the opposite active surface of that plate, and the contacted electrode. The ports that penetrate the thickness of one of the plates, are angled ports, such that the fluid flowing from one side of plate to the opposite side is not directed against any perpendicular surfaces. That is, the surfaces of the ports are not perpendicular to the plane of the plates, but are angled and/or curved to reduce turbulence and pressure loss.
The non-active surfaces of adjoining separator plates in a fuel cell stack can thus cooperate to provide passageways for directing at least one of the reactant from a respective fuel or oxidant manifold to the appropriate electrodes. In cases where the non-active surfaces of two adjoining separator plates accommodate both the oxidant and fuel reactant streams, the fuel and oxidant reactant streams are, of course, fluidly isolated from each other. Coolant passages may also be conveniently provided between the non-active surfaces of adjoining separator plates.
In other words, the fluid port directs the fluid at an angle between 0 degrees and 90 degrees from the direction of fluid flow in the passageway directly upstream of the fluid path. In this disclosure, 0 degrees is defined as being parallel to the direction of the fluid flow in the passageway upstream of the fluid port and 90 degrees is defined as being perpendicular to the direction of the fluid flow in the passageway upstream of the fluid port. That is, the fluid port walls are shaped so that the fluid flow vectors are not directed against any walls that are angled more than 90 degrees from the fluid flow vector. Preferably the effective angle of the fluid port walls is between about 20 degrees and about 45 degrees with respect to the direction of the fluid flow in the passageway upstream of the fluid port.
In a preferred embodiment, the non-active surfaces of adjoining separator plates provide fluid passageways for both the fuel and oxidant streams, which are, of course, fluidly isolated from each other. That is, within the electrochemical fuel cell stack:
at least one of the fuel stream passageways traverses a portion of one of the adjoining non-active surfaces of a pair of the separator plates, and the at least one fuel stream passageway comprises a fuel fluid port fluidly connecting a portion of the fuel stream passageway on the non-active surface, with the active surface of one of the pair of plates;
at least one of the oxidant stream passageways traverses a portion of one of the adjoining non-active surfaces of a pair of the separator plates, and the at least one oxidant stream passageway comprises an oxidant fluid port fluidly connecting a portion of the oxidant stream passageway on the non-active surface, with the active surface of the other of the pair of plates; and
the fuel fluid port and the oxidant fluid port each comprise walls that are angled more than 0 degrees and less than 90 degrees with respect to the direction of fluid flow in the respective fuel and oxidant passageways upstream of the respective fluid port.
In preferred embodiments, to further reduce turbulence the fluid port walls are curved. For example, the fluid port walls may be convex. The fluid port walls may also be curved more than one direction. For example, the walls may be curved in both the in-plane and through-plane directions (wherein the xe2x80x9cplanexe2x80x9d is defined herein as the plane of the active surface).
In one embodiment the angled fluid port is in the shape of an elongated slot and is fluidly connected to a plurality of fluid channels formed in the active surface.
In any of the above embodiments, the separator plates may be flow field plates wherein the active surfaces have reactant flow field channels formed therein, for distributing reactant streams from the supply manifolds across at least a portion of the contacted electrodes. In such cases the angled fluid ports may fluidly connect passageways on the non-active plate surfaces to reactant flow field channels on the active plate surface.
Fuel cell separator plates incorporating the disclosed features may be made from any materials that are suitable for fuel cell separator plates. Preferred properties for fuel cell separator plate materials include impermeability to reactant fluids, electrical conductivity, chemical compatibility with fuel cell reactant fluids and coolants, and physical compatibility with the anticipated operating environment, including temperature and the humidity of the reactant streams. For example, carbon composites have been disclosed herein as suitable materials. Expanded graphite composites may also be suitable materials. The disclosed discrete fluid distribution channels may be formed, for example, by embossing a sheet of expanded graphite material. Composite plate materials may further comprise a coating to improve one or more of the plate""s desired properties. Persons skilled in the art will understand that the present separator plates may be made from other materials that are used to make conventional separator plates, such as, for example, metal.
When expanded graphite is the material selected for the separator plates, the fluid ports, in addition to other features of the plate, such as, for example, the fluid passageways, and flow field channels, may be made by embossing a sheet of deformable expanded graphite. A pair of embossing dies may comprise features that cooperate with one another to form features such as the fluid ports. When a carbon composite is the material selected for the separator plates, the fluid ports and other features such as the flow field channels and non-active surface passageways may be molded.
The electrochemical fuel cell stack may optionally further comprise reactant exhaust manifolds for directing a reactant stream from one, or preferably more, of the fuel cell electrodes. In preferred embodiments, reactant stream passageways fluidly interconnecting the reactant exhaust manifolds to the electrodes also traverse a portion of adjoining non-active surfaces of a pair of separator plates and comprise angled exhaust ports fluidly connecting the non-active surface of the plates to the respective active surfaces.
In further embodiments passages for a coolant may also be formed between co-operating non-active surfaces of adjoining anode and cathode plates, or one or more coolant channels may be formed in the active surface of at least one of the cathode and/or the anode separator plates. In an operating stack, a coolant may be actively directed through the cooling channels or passages by a pump or fan, or alternatively, the ambient environment may passively absorb the heat generated by the electrochemical reaction within the fuel cell stack.
As mentioned above, passageways for both the fuel and oxidant reactant streams may extend between adjoining non-active surfaces of the same pair of plates, but the passageways are fluidly isolated from each other. To improve the sealing around the reactant stream passageways located between adjoining non-active surfaces of the separator plates, the fuel cell stack may further comprise one or more gasket seals interposed between the adjoining non-active surfaces. Alternatively, or in addition to employing gasket seals, adjoining separator plates may be adhesively bonded together. To improve the electrical conductivity between the adjoining plates, the adhesive is preferably electrically conductive. Other known methods of bonding and sealing the adjoining separator plates may be employed.
In any of the embodiments of an electrochemical fuel cell stack described above, the manifolds may be selected from various types of stack manifolds, for example internal manifolds comprising aligned openings formed in the stacked membrane electrode assemblies and separator plates, or external manifolds extending from an external edge face of the fuel cell stack.
As used herein, adjoining components are components that are in contact with one another, but are not necessarily bonded or adhered to one another. Thus, the terms xe2x80x9cadjoinxe2x80x9d and xe2x80x9ccontactxe2x80x9d are intended to be synonymous.